The process and apparatus set forth herein generally relates to turbomachinery such as compressors or turbines having bearings utilizing active magnetic technology and more specifically to improving the sealing arrangement of the turbomachinery shaft.
Active magnetic technology in the form of electromagnetic bearings is currently utilized in some turbomachinery to levitate rotors and shafts, instead of conventional technologies like rolling element bearings or fluid film bearings. The position of the rotor is monitored by position sensors. Usually there are at least 4 position sensors equally spaced around the bore of the magnetic bearings. These sensors feed the information to a controller, which in turn adjusts the electrical current supplied to the electromagnetic bearings to maintain the center of the shaft at a desired position or within a desired tolerance range. The bearing electronics normally seeks to keep the shaft at equal distance from all the sensors. In this position, the shaft axis and the electromagnetic bearings axis are substantially coaxial. Substantially coaxial means that the shaft can deviate from the axis of the electromagnetic bearings by an allowable tolerance that does not affect the operation of the turbomachinery but which can vary depending upon the design of the turbomachinery, and upon various deviations from an ideal situation, such as unbalance, run-out or unsteady aerodynamic forces. As used herein, the normal operating position of the shaft is also referred to as the centered position, meaning that the shaft axis coincides with (or lies within an acceptable tolerance of) the bearing axis. As turbomachinery normally includes at least two sets of radial bearings, here electromagnetic bearings, the descriptions set forth herein apply to each of the sets of bearings.
In the event of a loss of power to the bearing electronics during rotation, or of a failure of the control electronics, the magnetic bearings are disabled, and the shaft can no longer be supported by the electromagnetic bearings. The shaft would then be supported by the mechanical components of the electromagnetic bearings supplied for this purpose and/or supported by seals. These elements and the shaft are not designed for permanent mechanical contact, particularly when the turbomachinery is rotating. Therefore, mechanical bearings are provided as a back-up or safety feature to support the shaft when the magnetic bearings are disabled. The mechanical bearings are fixed with respect to the electromagnetic bearing, and both concentric and coaxial with them as permitted by manufacturing tolerances. Therefore, as used herein, this common center of the electromagnetic and mechanical bearings is referred to as the center of the bearings meaning that the electromagnetic and mechanical bearings are both concentric and coaxial. When the magnetic bearings are disabled, the shaft comes to rest under the effect of gravity and other static forces that may be present. When the axis is oriented horizontally in the turbomachinery, the rest position will normally be the lowest position within the allowable clearance of the mechanical bearings. When the axis is vertical, the rest position is not predictable. While the clearance between parts such as shafts and bearings will vary dependent on equipment size, a radial clearance between a shaft and electromagnetic bearing for a typical centrifugal compressor is of the order of about 1 mm (0.040 inches). During normal or powered operation of the turbomachinery, the rotating machinery must operate without contacting the mechanical bearings to avoid wear of both the shaft and the bearings, while the mechanical bearings remain stationary. Thus, there must be some clearance between the shaft and the mechanical back-up bearings when the shaft is magnetically levitated. The radial clearance between the shaft and the mechanical bearings usually is about 0.25 mm (0.010 inches). When the electromagnetic bearings are disabled, the mechanical bearings support the shaft while the turbomachinery is stopped or coasting to a stop, without any contact between the shaft and the electromagnetic bearings. While any one of a variety of bearings may be used as the back-up or safety bearings, rolling element type bearings are often preferred.
In turbomachinery, the shaft is also generally associated with gas seals to reduce or prevent any leakage along the shaft. For centrifugal compressors, gas leakage is also reduced or prevented at the inlet of the impellers for units utilizing shrouded impellers. The use of mechanical bearings as back-up bearings affects the design and mechanical arrangement of the seals used to reduce gas leakage in turbomachinery. The most straightforward design for seals is to have a cylindrical sleeve facing the shaft with low clearance. An alternative design used to reduce the leakage flow along the shaft is a seal having a labyrinth geometry, also referred to as a labyrinth seal. Rather than trying to seal with a single long barrier, a labyrinth seal uses multiple throttling steps to accomplish a reduction in leakage flow.
Gas seals are usually made of two hard surfaces with carefully matched diameters and geometries such that they do not contact adjutant surfaces in normal operation. Not only is mechanical friction between shaft and seal reduced or eliminated, but more importantly so is seal and shaft wear. As noted above, the simplest and best way to reduce gas leakage is to minimize the clearance between the seal and the shaft while simultaneously avoiding contact. A typical desired radial clearance for gas seals is 0.1 to 0.15 mm (0.004-0.006 inches) between the shaft and the seal, which is less than the typical radial clearance of the back-up mechanical bearings, typically 0.25 mm (0.010 inches). Having a clearance of the mechanical bearings greater than the desired clearance of the seals complicates the design of such seals. The design of the seals must consider the effect of the mechanical bearings. If the seals are rigidly fixed to the housing adjacent the shaft, the seal clearance should be at least equal to the clearance between the shaft and the mechanical bearings to avoid contact between the seal and the shaft when the magnetic bearings are disabled and the mechanical bearings are relied upon to provide the support for the shaft. Failure to provide this clearance could result in wear of the seal, which can result in loss of efficiency of the machine. In more severe cases, damage to the shaft can occur and overheating of the contacting parts, the shaft and the seal, can occur due to high temperatures resulting from friction. In addition, because of the difficulty associated with properly aligning the seals with the shaft, the assembly of fixed seals adjacent the shaft is difficult and time consuming.
One way to reduce the clearance between the shaft and seal is to mount the seal so that it is not rigidly mounted to the housing. Rather, the seal is movably mounted in such a way that it can slide radially so as to follow the motion of the shaft with limited resistance when the shaft position interferes with the seal. The use of springs to urge seal movement and substantially self-center the seal with respect to the shaft is well known. In such designs, when the power to the electromagnetic bearings is turned on and off, the seal follows the movement of the shaft inasmuch as the shaft interferes with the seal position; but the shaft and seal are not necessarily concentric, and may remain in contact. For instance, for a horizontally-oriented shaft, when power is removed from the electromagnetic bearings, the shaft drops onto the bearings as a result of gravity, until it contacts the back-up mechanical bearings. The seals are then biased downward against the shaft in its rest position, but the seals and the shaft are then in mutual contact and not concentric. Some wear will occur between the shaft and seals if the shaft is rotating in this position, such as when the shaft is coasting to a stop during power down. When power is restored to the electromagnetic bearings, the shaft is levitated back to its normal operating position and pushes the seals upward, the seals being in residual contact with the shaft. The seals still being in contact with the shaft, wear will occur when the machine is rotated in this position.
Although the contact forces are small when the electromagnetic bearings are re-levitated on restoration of power, there is still friction between the shaft and seal on reinstitution of rotation of the turbomachinery. The design of the seal must accommodate this friction. The materials must be selected to withstand this friction without excessive wear and heating, which makes the design more complicated and expensive. Despite these design efforts, the seal will eventually exhibit some amount of wear and performance deterioration. Periodic maintenance may be required to restore the operating characteristics of the turbomachinery, which may require seal replacement and/or shaft repair or replacement, unless additional wearing sleeves are provided in the design to protect the shaft, in which case replacement of these sleeves may be required during maintenance.
What is needed is a system that allows minimal clearance between the seal and shaft without the need for costly and intricately designed seals. A minimal clearance should be maintained during assembly and operation of the machine. The present invention provides methods that satisfy one or more of these needs and provides other advantageous features.